Numerous references describe tools located above a drill bit in a drillstring for periodically interrupting all or most of the drilling fluid flow to the bit. These tools fall into three general categories, based on their intended application. In the first category are hammer drills that periodically divert drilling fluid flow to reciprocate the drill bit against the bottom of the borehole. This concept was first presented by Wolski in his 1902 U.S. Pat. No. 699,273. More recent developments in downhole hammers by SDS Pty. Ltd. and Novatek Inc. are described in U.S. Pat. No. 5,803,188 (McInnes, 1998); U.S. Pat. No. 5,396,965 (Hall et al., 1995); and U.S. Pat. No. 5,222,425 (Davies, 1993). The second category includes measurement-while-drilling (MWD) systems that interrupt fluid flow to the bit to generate mud pulses in the fluid column to facilitate telemetry signals transmitted from the downhole equipment to receiving systems on the surface. An early form of this type of system is described by Jakosky in U.S. Pat. No. 1,963,090 (1934). Many patents have been granted since then that utilize mud pulse telemetry in some form. The third category of tools interrupt flow to the bit causing pressure fluctuations in the borehole at the bit face that enhance drilling efficiency. It is clear that the third category of tool provides a substantial benefit, and it would be desirable to provide further apparatus and a method based on interrupting flow to the bit to generate pulses so as to enhance drilling efficiency.
The benefits of interrupting all or most of the drilling fluid flow to the bit for the purpose of creating pressure fluctuations or pulses in the borehole are well understood and are described in references such as those noted above. These benefits relate to the following points:                When the pressure below the bit rapidly decreases to less than the rock pore pressure, a brittle rock formation is encouraged to fracture due to the differential pressure across the surface of the borehole;        A reduced pressure below the bit produces a downward force on the bit that increases the load on the cutters, improving their cutting efficiency; and        Rapidly changing pressures produce a “water hammer effect” or impulse that is transmitted to the drill bit and its cutters to also improve the cutting efficiency and fracturing of the rock by the bit.        
The following list includes brief descriptions of some of the more significant patents that describe using drilling fluid pulses to enhance drilling.    1. In U.S. Pat. No. 3,648,789 (1972), Chenoweth describes a hydraulic pulse generator that uses a shuttle valve to direct drilling fluid either up and out to the annulus or down to the bit. The shuttle valve changes position rapidly due to “the pressure pulse waves generated in the passages between its upper and lower positions.”    2. In U.S. Pat. No. 4,817,739 (1989), Jeter describes a “drilling fluid pulse generator for use above a drill bit to produce pulsations in drilling fluid flow.” Jeter's pulse generator valve is auto-cycling at a frequency determined by a spring-mass system and the system pressure variations caused by the opening and closing of the valve.    3. In U.S. Pat. No. 6,053,261 (2000), Walter describes a flow pulsing tool that uses a spring-mass system wherein a poppet periodically blocks drilling fluid flow to the bit, creating pressure pulses above and below the valve that travel at the speed of sound in the fluid.
In each of the tools described in the above-noted references, oscillation of the pulse generator valve is caused by pressure fluctuations in the tool, usually enhanced by the action of a spring. Another class of pulse generator tool can be described as a pilot-operated poppet valve. In a pilot-operated valve, fluid drives a pilot valve that controls the action of a main poppet valve, which provides a more positive valve action that is self-starting and regulated by the timing of fluid ports in the valve. The use of this type of valve to produce negative pressure pulses in the borehole is described in commonly assigned U.S. Pat. No. 6,237,701 (2001), in which Kollé et al. describe various embodiments of a pilot valve/poppet valve based downhole hydraulic impulse generator for borehole applications, the disclosure and drawings of which are hereby specifically incorporated herein by reference. The primary benefits of the hydraulic impulse generator are associated with the rapid reduction in borehole pressure under the bit. The benefits of this negative pressure pulse for drilling as described in U.S. Pat. No. 6,237,701 include:                Increased rate of penetration;        Early identification of potential gas kicks;        Downhole seismic signal generation while drilling;Additional applications of the negative pressure pulse in borehole applications other than drilling include:        De-scaling of tubulars; and        Formation cleaning.The rapid reduction of borehole pressure that occurs in the invention described in this patent is accomplished by providing a flow of low compressibility fluid, such as water or drilling fluid, through a conduit in the borehole and momentarily blocking the fluid flow with a pilot-operated poppet valve that reciprocates between open and closed positions. If the poppet valve closes in a time that is equal to or shorter than the two-way travel time of an acoustic wave in the annulus between the conduit and borehole, a negative impulse pressure is generated in the borehole beneath the discharge of the conduit. The annular flow passage may be restricted to increase flow velocity in the annulus and increase the magnitude of the resulting negative impulse pressure. If the tool is used for drilling, the length of the restricted flow area may be limited to be less than 1.5 meters to reduce torque. In this case, the two-way travel time of an acoustic pressure pulse in the restricted flow annulus is about 2 milliseconds. The poppet must therefore close completely in less than 2 milliseconds for the tool to be completely effective. The poppet valve is dynamically unstable; when closed, it is energized to open, and when open, it is energized to close. A pilot spool directs drilling fluid to either side of the poppet spool to energize it. The pilot spool is also dynamically unstable. As the valve oscillates between open and closed positions, the passages in the poppet spool direct drilling fluid to either end of the pilot spool to energize it from one position to the other. The pulse generator valve self-starts from any position and runs at a frequency determined by the flow rate of drilling fluid through the valve mechanism.        
One embodiment of the valve disclosed in U.S. Pat. No. 6,237,701 is incorporated in a drillstring within a housing including high speed flow courses. The valve closes in about one millisecond. Valve closure stops the flow of drilling fluid through the bit and through high speed flow courses in the housing around the bit. Stopping the upwards flow of drilling fluid through the flow courses generates a negative pressure pulse around the drill bit. This patent discloses that the valve closing time must be less than the two-way travel time of a pressure wave in the flow courses so that an intense negative pressure is generated below the bit. The valve disclosed in U.S. Pat. No. 6,237,701 can provide pulse amplitudes of from about 500 psi to about 1500 psi, with a cycle rate of from 15 to 25 times per second.
Although the relative locations of the pilot and poppet spools are not claimed with specificity in U.S. Pat. No. 6,237,701, a preferred embodiment described therein and early working models are configured with the pilot and poppet spools vertically in-line and physically separated from each other in interconnected housings. The in-line configuration requires multiple long intersecting passages to carry drilling fluid to and from the pilot and poppet spools. Transverse cross-port passages are required for interconnecting the various axial fluid passages. These cross-port passages are plugged from the outside to seal the internal pressure. Multiple sealing elements are required to seal the interconnecting fluid passages between housings.
While functional, the in-line configuration is extremely complex and is correspondingly difficult to manufacture and assemble. The housings are difficult to align, and the seal elements between the housings are prone to premature failure, particularly in the unforgiving environment associated with drilling operations. The long, interconnecting fluid passages and cross-drilled holes are subject to rapid erosion by the drilling mud at each change of flow direction. The valve is also subject to large pressure drops due to fluid friction through the long, complex passages. It would thus be desirable to provide a pilot valve/poppet valve based downhole hydraulic impulse generator for enhancing oil and gas drilling that does not suffer from the disadvantages of the embodiment described in U.S. Pat. No. 6,237,701.
Oil and gas casing and production targets are commonly determined by reference to seismic data. These data are conventionally obtained by conducting seismic reflection and refraction studies using surface sources such as vibrator trucks or air guns. Such sources create pressure waves in the earth that travel at different speeds, depending upon properties of the strata such as density and porosity. The vertical scale in a seismic image is measured in terms of seconds of travel time for the seismic waves. To be useful for planning drilling operations, these seismic images must be depth-corrected using assumptions regarding the velocity of seismic waves. The computed depth to a seismic target may differ from the actual depth by 20 percent or more.
If a well borehole is available in the vicinity of a seismic test, depth correction information can be obtained from a check shot survey, in which a seismic receiver is placed in the well and the travel time from a surface source to the receiver is observed. A vertical seismic profile (VSP) of velocity is obtained by moving the receiver to various depths in the well. A reverse vertical seismic profile (rVSP) provides the same information by placing the source in the well and a receiver on the surface. Conventional seismic profiling requires that drilling stop while the survey is carried out.
A tricone drill bit can provide the seismic source for rVSP in real time, allowing continuous depth correction of seismic profiles, as described by W. H. Borland in an article (Butsuri Tansa (1988) 51:1). Two seismic-while-drilling (SWD) systems, (Tomex from Baker-Atlas, and DBSeis from Schlumberger), rely upon the seismic energy generated by tricone bits, as described by J. W. Rector and B. P. Marion in “The use of drill bit energy as a downhole seismic source” (Geophysics (1991) 86:5). The bit creates acoustic noise as it bounces and scrapes against the rock. The acoustic signal is transmitted through the drillstring to the surface, where it is recorded by an accelerometer or other receiver. The drillstring signal is cross-correlated with signals received by geophones on the surface to create a seismic record. Current SWD techniques, which employ the drill bit as a source, provide seismic profiles that are helpful in detecting abnormal pressure trends, but do not provide a look-ahead capability. U.S. Pat. No. 5,191,557 (Rector et al., 1993) describes enhanced signal processing that is required to use a rig reference sensor with a drill bit seismic source for VSP and seismic imaging. While SWD systems can provide acoustic data while drilling, it would be even more desirable to provide a drill tool that can both enhance drilling performance and act as a seismic source during the drilling operation.
Drag bits (representing about 80 percent of offshore drilling) do not create a useable seismic signal, while tricone bits do not create a useful signal in soft formations. Furthermore, roller cone bits only produce a dipole radiation pattern along the axis of the drillstring, which limits the placement of seismic receivers to locations near the drill rig (a seismically noisy area due to pumps and other rig activity) and restricts the use of bit seismic techniques to vertical wells where the target formations occur in planes perpendicular to the borehole axis. It would be desirable to provide a drill tool that can both enhance drilling performance and act as a seismic source in which the seismic radiation pattern produced by the tool is not so limited.
Deep drilling operations are subject to blowouts when formation pressures become greater than the pressure of drilling fluids in the borehole. Methods for determining pore pressure ahead of the drill bit presently rely upon interpretation of seismic reflection data. Increased pore pressure causes a reduction in compression wave velocity, so VSP techniques can be used to identify abnormal pore pressure trends in a formation. This procedure also requires drilling to be stopped. SWD, using the drill bit as a source, has been attempted for imaging formations ahead of the bit. In many formations and under common operating conditions, tricone bits do not generate a signal-to-noise ratio that is usable for SWD. In particular, the drill bit seismic signal is limited to relatively low frequencies (under 80 Hz) and is incoherent, requiring significant post-processing. At a frequency of 80 Hz, the depth resolution in a 3 km/s formation is 37.5 meters (approximately four 9-meter joints of drill pipe), which is not useful to drillers.
Those skilled in the art will recognize that an ideal seismic source for profiling, reflection imaging, or refraction studies should be a point source and have a broad bandwidth. A broadband signal may be generated by a single impulse source, by sweeping a sinusoidal source over a broad range of frequencies, or by generating multiple impulses with a cycle period that varies over a full octave. It would be desirable to provide a drill tool that can both enhance drilling performance and act as a seismic source for SWD, providing a broad range of frequencies, to more readily facilitate the imaging of formations ahead of the bit.
The use of a swept impact seismic technique for surface applications using a mechanical impact tool with a variable cycle rate has been suggested in the prior art (Park, C. B., Miller, R. D., Steeples, D. W., and Black, R. A., 1996, Swept Impact Seismic Technique (SIST) Geophysics, 61 no. 6, p. 1789–1803). Varying the rate of a pure impulse signal over a full octave generates a continuous broadband signal. The received signal can be cross-correlated with the impact signal to generate a seismic record with high signal-to-noise ratio. The signal-to-noise ratio can be increased substantially by operating the source over a long period of time. U.S. Pat. No. 6,394,221 (Cosma, 2002) discloses a technique and apparatus for generating a swept impact axial or radial load at the bottom of a borehole using an electrically actuated hammer. This tool is designed to be clamped in a borehole at various depths for seismic profiling.
A number of references disclose variable frequency downhole seismic sources. For example, U.S. Pat. No. 4,033,429 (Farr, 1977) describes a drillstring with a sleeve containing a helical pattern of holes that periodically align with holes in the drillstring. Rotation and translation of the string through the sleeve create a signal that sweeps over a broad range of frequencies up to 80 Hz, depending on the drillstring rotation speed. Significantly, the apparatus described in the Farr patent requires an interruption in the drilling process to actuate the tool. U.S. Pat. No. 6,094,401 (Masak et al., 2000) describes the use of a downhole MWD mud pulse telemetry system to generate a sinusoidal frequency sweep over a range of frequencies from 1 to 50 Hz. Masak's device uses an electric motor to drive a rotor at variable rotation rates. The rotor interacts with a stator to restrict the mud flow to the bit. Restricting the flow generates axial shaking loads of up to 3000 lbf. These loads are transmitted through the bit to the formation. The coupling between the bit and the formation is limited by the relative axial stiffness of the drillstring and the reference discloses the use of a thruster subassembly to increase coupling. As with drill bit seismic, axial shaking of the drillstring generates primarily a dipole signal that propagates along the borehole axis. Seismic receivers must therefore be located near the drill rig, which is a source of substantial masking of seismic noise.
A number of options have been studied for generating a strong seismic signal while drilling. Most options involve stopping the drilling process to actuate a downhole source such as a piezoelectric vibrator, hydraulic or mechanical jarring tools, or dropping the drillstring. All of these options interrupt the drilling process and increase the potential for borehole instability. Frequent drilling interruptions would not be an acceptable practice for most operators.
Prior art SWD techniques result in low signal-to-noise ratios, and the resulting signals require substantial processing and interpretation. It would be desirable to provide a broadband high-amplitude SWD source that enables unambiguous real-time interpretation of formation velocity and reflections ahead of the bit.